Boron-added high strength steel for bolt and high strength bolt having excellent delayed fracture resistance

Abstract

Provided are: a boron-added high strength steel for bolt excellent in delayed fracture resistance even having a tensile strength of 1100 MPa or more without addition of large amounts of expensive alloy elements such as Cr and Mo: and a high strength bolt made from the boron-added high strength steel for bolt. The high strength steel for bolt contains C of 0.23% to less than 0.40%, Si of 0.23% to 1.50%, Mn of 0.30% to 1.45%, P of 0.03% or less (excluding 0%), S of 0.03% or less (excluding 0%), Cr of 0.05% to 1.5%, V of 0.02% to 0.30%, Ti of 0.02% to 0.1%, B of 0.0003% to 0.0050%, Al of 0.01% to 0.10%, and N of 0.002% to 0.010%, with the remainder being iron and inevitable impurities. The steel has a ratio ([Si]/[C]) of the Si content [Si] to the C content [C] of 1.0 or more and has a ferrite-pearlite mixed microstructure.

The invention claimed is: 
     
       1. A high strength bolt, obtained by a process comprising:
 forming a bolt-shaped workpiece using a steel, the steel comprising: by mass %,
 from 0.23% to less than 0.40% of C; 
 from 0.23% to 1.50% of Si; 
 0.03% or less, excluding 0%, of P; 
 0.03% or less, excluding 0%, of S; 
 from 0.05% to 1.5% of Cr; 
 from 0.02% to 0.30% of V; 
 from 0.02% to 0.1% of Ti; 
 from 0.0003% to 0.0050% of B; 
 from 0.01% to 0.10% of Al; and 
 from 0.002% to 0.010% of N, 
 with the remainder being iron and inevitable impurities; 
 wherein the steel has a ratio [Si]/[C] of 1.0 or more, wherein [Si] and [C] represent a content of Si and a content of C, respectively; and 
 wherein the steel has a mixed microstructure of ferrite and pearlite; 
 subjecting the bolt-shaped workpiece to a quenching treatment; and 
 subjecting the bolt-shaped workpiece after quenching to a tempering treatment, 
 wherein the bolt has a tensile strength of 1100 MPa or more, and 
 wherein a VI value is 10% or more, where the VI value is determined from a V content in precipitates having a particle size of 0.1 μm or more and a V content in the steel and specified by the following Expression (1):
     VI  value(%)=[( V  content in precipitates having a particle size of 0.1 μm or more)/( V  content in the steel)]×100  (1).
 
 
 
 
     
     
       2. The high strength bolt of  claim 1 , wherein an austenitic grain size number of a bolt shank after quenching and tempering is 8 or more. 
     
     
       3. The high strength bolt of  claim 1 , wherein the bolt-shaped workpiece is subjected to a quenching treatment while heating the bolt-shaped workpiece to 850° C. to 920° C. 
     
     
       4. The high strength bolt according to  claim 1 , the steel further comprises, by mass %, 0.10% or less, excluding 0% of Mo.